1. Field of the Invention
The present invention relates to a high frequency signal attenuator, and more specifically relates to a miniaturized attenuator in which an attenuation of a passing signal is limited to 4 to 5 dB in order to enable use in combination with an amplifier having gain dispersion and gain temperature characteristics.
2. Description of the Related Art
When incorporating an amplifier into a system, it is desirable that particularly gain has uniform characteristics in a specified temperature range or between chips. This is because a level diagram in the entire system can be maintained constant. When the gain of the amplifier is reduced, an output of the amplifier reduces and a following circuit cannot be driven. Moreover, when the gain of the amplifier is increased, a final-stage transistor in the amplifier is brought into an over input state, and thus there is a possibility that a problem of reliability arises.
In order to make the gain uniform between the chips, there is a method of adjusting a bias to be applied to a transistor so as to maintain the gain constant. For example, in the case of a two-stage amplifier, a gate voltage and a drain voltage at a first stage or a gate voltage and a drain voltage at a second stage are adjusted so that the entire gain is adjusted.
However, this method causes (1) a problem that a power consumption changes and (2) a problem that in the case where the amplifier is configured at more stages, an adjustable range is limited because an output level and input levels of the transistors at respective stages as well as the gain should be considered. In the case where there is a plurality of amplifiers, the adjusting methods become fairly complicated so that a throughput of manufacturing is lowered.
Another method is a method of combining an attenuator with an amplifier so as to form a system. Typical one of such a method is a method of connecting resistors corresponding to gain dispersion of an amplifier in series. This method can reduce the gain dispersion of the amplifier, but it is difficult to compensate temperature characteristics of the gain.
Publicly known examples of the attenuator will be cited below.
There is an example including a lange coupler having four ports and a plurality of series circuits configured by FET and resistor which are connected in parallel to two ports of the coupler (for example, see Patent Document 1).
There is an example which π type and T type resistor attenuating circuits are combined complicatedly in order to obtain an attenuating ability with high accuracy (for example, see Patent Document 2.
There is an example which two series circuits, each configured by FET and resistor, are connected in parallel at an inlet side and an outlet side of a line having λ/4 wavelength (for example, see Patent Document 3).
There is an example which a second gate terminal is connected to a drain in a dual gate transistor in order to improve nonlinearity generated by inserting an attenuator (for example, see Patent Document 4).
There is an example which a bias voltage is applied to a drain of a transistor via a choke coil in order to improve nonlinearity generated by inserting an attenuator (for example, see Patent Document 5).
There is an example which a transistor for bias is connected to a gate of a transistor for amplification and a gain control voltage is supplied to a gate of the transistor for bias so that linearity is not deteriorated even if the gain is varied by a high frequency amplifying circuit (for example, see Patent Document 6).
There is an example which has a complicated circuit configured by plural-stage transistors and plural-stage attenuators in order not to damage linearity and in order to obtain a large gain variable width even if the gain is varied by a high frequency amplifying circuit (for example, see Patent Document 7).
There is an example which one end of a high impedance line is connected with a microwave line and a PIN diode as a variable impedance element is connected with the other end in order to attenuate an over input (for example, see Patent Document 8).
Patent Document 1
Japanese Patent Application Laid-Open No. 06-232607 (1994) “ATTENUATOR” (Paragraph No. [0008], FIG. 1).
Patent Document 2
Japanese Patent Application Laid-Open No. 06-112767 (1994) “LOW-LOSS ATTENUATOR TO BE SWITCHED” (claim 1, FIG. 2).
Patent Document 3
Japanese Patent Application Laid-Open No. 07-312508 (1995) “VARIABLE ATTENUATOR” (paragraph No. [0019], FIG. 1).
Patent Document 4
Japanese Patent Application Laid-Open No. 50-97252 (1975) “VARIABLE RESISTANCE DEVICE” (claims, FIG. 4).
Patent Document 5
Japanese Patent Application Laid-Open No. 06-77762 (1994) “VARIABLE ATTENUATOR” (paragraph No. [0005], FIG. 1).
Patent Document 6
Japanese Patent Application Laid-Open No. 10-261925 (1998) “HIGH FREQUENCY AMPLIFIER” (paragraph No. [0020], FIG. 1).
Patent Document 7
Japanese Patent Application Laid-Open No. 2001-102882 “HIGH FREQUENCY AMPLIFIER” (paragraph Nos. [0024] through [0031], FIG. 1).
Patent Document 8
Japanese Patent Application Laid-Open No. 2001-244706 “VARIABLE ATTENUATOR” (paragraph Nos. [0009] through [0031]FIG. 1).
In the patent documents 1 and 2, although an accurate attenuation value can be obtained, the circuit configuration is complicated and miniaturization is difficult. Also in the patent document 3, since the example requires a line with specified wavelength and a four-system attenuation circuit, miniaturization is difficult, and since the attenuation circuits are provided separately, an L component is included and thus it is difficult to obtain a constant attenuation value with wide band. In the patent documents 4 through 7, since the examples require a bias power source, a choke coil and a complicated attenuator, miniaturization is difficult. The example in the patent document 8 is a limitter circuit, and it cannot obtain a constant attenuation value regardless of an input level.